As Jeremy mentioned, I bought one of the 2.5# jugs several years ago and have been using it since without any problems. I only use it occasionally (5-8 granules sprinkled a couple of times a year). It seems to help with the slugs, but I never get rid of them completely. I am hesitant about always having some in the tank, but possibly with regular application over a several month period I could probably break the breeding cycle.
I also keep fruit in my tanks as a magnet for flies and to allow fly breeding. Apples and oranges really seem to attract the slugs, so I will also pull them off manually when I see them. Lettuce is supposed to work well also - but they eat it quite fast - and more often than not I just end up feeding them...